Receptor tyrosine kinases are important regulatory proteins that control many aspects of cellular metabolism, growth, and differentiation. Many extracellular signaling molecules are known to exert their effects on cell regulation by binding to these receptors. The ultimate objective of the proposed research is to understand the molecular mechanisms that receptor tyrosine kinases use to regulate cellular behavior. This could provide important insights into how these cellular processes are regulated and coordinated, and how inappropriate activation of receptor tyrosine kinases can lead to neoplastic transformation of cells. In order to address this question, we study the differentiation of a photoreceptor cell in the Drosophila eye that is regulated by a receptor tyrosine kinase, the product of the seven less gene. Activation of the seven less receptor results in a single cell to develop as a R7 photoreceptor rather than a non-neuronal cell. The activation of Sevenless serves as a switch that induces expression of the phyllopod gene in the presumptive R7 cell. phyllopod encodes a nuclear protein required for fate determination of the R7 photoreceptor. In previous work, we studied the seven in absentia (sina) gene and found that it also plays a central role in the decision process that is regulated by Sevenless. sina encodes a nuclear protein that is expressed ubiquitously in the eye. Our studies have demonstrated that the Sina and Phyllopod proteins specifically associate to form a complex in vitro. This suggests that both proteins directly interact in presumptive R7 cells, and that the complex is active in regulating R7-specific gene expression. We have conducted a genetic screen for mutations that attenuate sina activity and identified 33 genes. We have characterized one of these genes in detail. The prospero gene becomes transcriptionally activated at a low level in all Sevenless-competent cells prior to Sevenless signaling. This requires activation of the EGF receptor tyrosine kinase. Restriction of high-level prospero transcription to R7 cells appears as a subsequent event, which requires Sevenless activation. Our results suggest that both transcriptional responses are linked to activation of the Ras signal transduction pathway that operates downstream of both receptor tyrosine kinases. The aims of the proposed research are to: (l) biochemically characterize Sina and Phyllopod proteins; (2) conduct molecular characterization of two more genes; (3) uncover the molecular mechanism by which receptor activation of Ras 1 results in two distinct transcriptional responses by the prospero gene.